PURPOSE: To correct the displace command value of a piston rod on the basis of a position deviation amount, while detecting correctly the position deviation of the piston rod generated on the position deviation amplifier of a hydraulic cylinder control device when the zero point shift of a spool is generated by change of oil temperature.

CONSTITUTION: An oil pressure servo system is formed in such a way that a hydraulic cylinder control device 35 is provided with a position deviation amplifier for taking-in a detected position deviation amount while detecting by a first position detecting sensor 33 the position deviation amount of a piston rod 13 generate when a zero point shift is generated on the basis of a real spool displacement amount equivalent to the fluctuation of the oil temperature of the spool detected by a second position sensor 31, and a correcting means for correcting a piston rod displacement command value on the basis of the position deviation amount taken in the position deviation amplifier.